1. Field of the Invention
The present invention relates to electrodes, electrochemical elements (e.g., oxygen pumps, electromotive force generation elements) and gas sensors. This invention also relates to gas measurement methods.
2. Description of the Related Art
It has been an important subject to reduce harmful gas components that are included in exhaust gas, which is emitted from various combustion apparatuses or combustion engines, as of automobile engines and boilers. In order to decrease the harmful gas components in exhaust gas, the combustion apparatuses or exhaust gas-purifying devices need to be controlled or monitored (e.g., catalyst deterioration has to be detected). In order to control or monitor the apparatuses, detecting devices that are capable of measuring concentrations of oxygen gas, flammable gas (e.g., various hydrocarbon gases), and nitrogen oxide gas (NOX gas) are required. Oxygen-pump for expelling oxygen gas from a gas detection chamber or introducing oxygen gas into the gas detection chamber is very useful for assembling the detecting devices. Known flammable gas sensors use an oxide-ion conductive solid electrolyte (e.g., yttria-stabilized zirconia (YSZ)) for forming at least a part of a wall surrounding a gas detection chamber. Voltage detecting type and current detecting type are known.
U.S. Pat. No. 5,879,525 teaches the current detecting type flammable gas sensor. As schematically shown in FIG. 25, the gas sensor includes oxide-ion conductive solid electrolyte 264, a pair of electrodes 252, 254, and a pair of electrodes 258, 260. Oxide-ion conductive solid electrolyte 264 (e.g., YSZ) forms a wall surrounding gas detection chamber 250. A pair of electrodes 252, 254 is provided at opposite faces of oxide-ion conductive solid electrolyte 264. Also, a pair of electrodes 258, 260 is provided at opposite faces of oxide-ion conductive solid electrolyte 264. A part of oxide-ion conductive solid electrolyte 264 and a pair of electrodes 252, 254 form first oxygen pump 256. A part of oxide-ion conductive solid electrolyte 264 and a pair of electrodes 258, 260 form second oxygen pump 262. First oxygen pump 256 adopts Pt electrode or Pt—Au electrode 254 (electrode that includes Pt or Pt—Au and a ceramic substance) within gas detection chamber 250.
At first, mixture gas of oxygen gas and flammable gas is introduced within gas detection chamber 250. Then electric voltage is applied between electrodes 252, 254 of first oxygen pump 256. While first oxygen pump 256 is being activated, oxygen gas existed within gas detection chamber 250 is expelled to the outside. Gas detection chamber 250 becomes almost free of oxygen gas after first oxygen pump 256 is activated. Next, second oxygen pump 262 is activated by applying voltage between electrodes 258, 260 of second oxygen pump 262. While second oxygen pump 262 is being activated, oxygen gas is introduced to gas detection chamber 250 and flammable gas is oxidized with the introduced oxygen gas. A quantity of oxygen gas introduced for oxidizing flammable gas within gas detection chamber 250 is proportional to value of current flowing through second oxygen pump 262.
In this gas sensor, the Pt electrode or Pt—Au electrode 254 is disposed within gas detection chamber 250. However, the Pt electrode and Pt—Au electrode have activity not only to oxygen gas but also to flammable gas (various hydrocarbons, etc). This causes oxidization of flammable gas while first oxygen pump 256 is activated. That is, a portion of flammable gas is oxidized while first oxygen pump 256 is activated for expelling oxygen gas from gas detection chamber 250. Consequently, the second oxygen pump 262 introduces less quantity of oxygen gas than a quantity of oxygen required to oxidize flammable gas that were included within gas detection chamber 250 before first oxygen pump 256 was activated. As a result, second oxygen pump 262 cannot measure flammable gas concentration accurately.
Also, known nitrogen oxide gas sensors include first oxygen pump and second oxygen pump, both of which have oxide-ion conductive solid electrolyte. Some of the gas sensors use a Pt—Au electrode for forming first oxygen pump, and the Pt—Au electrode is disposed within gas detection chamber.
At first, mixture gas of oxygen gas and nitrogen oxide gas is introduced within gas detection chamber. Then electric voltage is applied between electrodes of first oxygen pump. While first oxygen pump is being activated, oxygen gas existed within gas detection chamber is expelled to the outside. Gas detection chamber becomes almost free of oxygen gas after first oxygen pump is activated. Next, second oxygen pump is activated by applying voltage between electrodes of second oxygen pump. While second oxygen pump is being activated, nitrogen oxide gas within gas detection chamber is decomposed (i.e., reductive reaction) into nitrogen gas and oxygen gas. Decomposed oxygen gas is expelled to the outside. When oxygen is conducted through oxide-ion conductive solid electrolyte of second oxygen pump, current flows between electrodes of second oxygen pump. A quantity of oxygen gas expelled by second oxygen gas is proportional to value of current flowing through second oxygen pump. Based on a quantity of oxygen gas decomposed from nitrogen oxide gas, nitrogen oxide gas concentration can be calculated.
In this gas sensor, the Pt—Au electrode of first oxygen pump disposed within gas detection chamber has activity only with oxygen gas and does not affect to nitrogen oxide gas within gas detection chamber, as long as the voltage applied to first oxygen pump is low. Accordingly, when the voltage applied to first oxygen pump is low, only oxygen can be pumped out and pumping by first oxygen pump provides with little influence upon the nitrogen oxide gas. However, when the voltage applied to first oxygen pump increases, the activity of the Pt—Au electrode to nitrogen oxide gas increases. As a result, accuracy of nitrogen oxide gas concentration measurement by second oxygen pump degrades.
In a case that both of flammable gas concentration and nitrogen oxide gas concentration within mixture gas of oxygen gas, flammable gas, and nitrogen oxide gas are required to be measured, oxygen gas within gas detection chamber must be expelled from gas detection chamber by activating oxygen pump. The electrode of the oxygen pump is required to have low activity to both flammable gas and nitrogen oxide gas. However, the aforesaid Pt electrode has activity to both flammable gas and nitrogen oxide gas. In the aforesaid Pt—Au electrode, the activity to the nitrogen oxide gas is low, however, the activity to flammable gas is high. As mentioned above, electrodes that have high activity to oxygen gas and low activity to flammable gas and nitrogen oxide gas have not been found yet. Thus, when a gas subject to gas concentration measurement contains flammable gas and nitrogen oxide gas, the known techniques cannot measure the concentrations of these gases with high accuracy.